Re: Doppler Distortion - Fact or Fiction?

From: zigoteau (zigoteau_at_ausi.com)
Date: 08/19/04 

Date: 19 Aug 2004 01:17:19 -0700

Bob Cain <arcane@arcanemethods.com> wrote in message news:<cfplp9010hq@enews3.newsguy.com>...

Hi again, Bob,

Where are you?
 
I'm adding another gloss on my previous two posts because I've found
the following reference to loudspeaker distortion caused by nonlinear
sound propagation in air which is particularly relevant:

http://www.tvhandbook.com/support/pdf_files/audio/Chapter5_5.pdf

The dominant nonlinearity is second order and can become appreciable
for horn loudspeakers. If a loudspeaker diaphragm of area S m^2
generates acoustic power W watts at a frequency f Hz, then

THD% = 7.5*sqrt(W)/S*(f/f_c)

f_c is the cutoff frequency, given for an exponential horn by

f_c = mC/4/pi Hz

where m is the the flare constant and C is the speed of change
(defined in the pdf file). Assuming that f_c is in the audible range,
this formula gives huge distortions for quite modest power fluxes.

The second-order nature of the nonlinearity fits into the variation
with pressure of the speed of sound and or the acoustic impedance that
I discussed in my previous posts, although I still haven't totally
grasped how the frequency dependence fits.

There are quite a few references on the web to the use of high-power
ultrasound demodulated by the nonlinearity of the air to produce a
localized source of audible sound, but so far I haven't found anything
giving specific equations and/or orders of magnitude.

Cheers,

Zigoteau.

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